This invention relates to the preparation of substantially binderless particles of zeolite L, particularly in the form of aggregates for use in catalysis, particularly for aromatisation.
Zeolite L has been known for some time as an adsorbant, and in U.S. Pat. No. 3,216,789 is described as an aluminosilicate of the formula: EQU 0.9-1.3 M .sub.2/n O:Al.sub.2 O.sub.3 :5.2-6.9.SiO.sub.2 :yH.sub.2 O
(where M is an exchangeable cation of valency n and y is from 0 to 9) having a characteristic X-ray diffraction pattern. The preparation of zeolite L is described in U.S. Pat. No. 3,216,789, EP-A-167755, EP-A-142355, EP-A-142347, EP-A-142349, EP-A-109199, PL-A-72149, U.S. Pat. No. 3,867,512, and SU-548567.
EP-A-96479 describes and claims zeolite L having a characteristic morphology and size, which is particularly valuable for use as a catalyst base in hydrocarbon conversions such as aromatisation, and comprising crystallites in the form of cylinders with a mean diameter of at least 0.1 micron, preferably at least 0.5 micron.
EP-A-96479 describes a synthesis of zeolite L which is conducted so that the amount of the contaminant zeolite W, which is know to grow in such a system as a competitive phase, is minimised. A preferred synthesis gel described in EP-A-96479 has the following mole ratios: EQU 2.62K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :16OH.sub.2 O
and it is discussed how this gel may be varied by changing the molar amount of one component within the following ranges:
______________________________________ K.sub.2 O: 2.4-3.0 moles Al.sub.2 O.sub.3 : 0.6-1.3 moles SiO.sub.2 : 8-12 moles H.sub.2 O: 120-240 moles ______________________________________
EP-A-142353, EP-A-142354 and EP-A-185519 describe developments of this process for forming cylindrical zeolite L.
Zeolite L may be used as a catalyst base in aromatisation reactions. U.S. Pat. No. 4,104,320 discloses dehydrocyclisation of aliphatic compounds in the presence of hydrogen using a catalyst comprising zeolite L and a group VIII metal. The particular zeolite disclosed in EP-A-96479 is remarkably effective in such aromatisation reactions being capable of forming catalysts which have extended lifetime. Such dehydro- cyclisation and/or aromatisation reactions and catalysts for use in such reactions are also described in EP-A-107389, EP-A-184451, EP-A-142351, EP-A-145289, EP-A-184450, U.S. Pat. No. 4,614,834, GB-A-2116450, GB-A-2114150, U.S. Pat. No. 4,458,025, U.S. Pat. No. 4,456,527, GB-A-2142648, GB-A-2106483, U.S. Pat. No. 4,443,326, GB-A-2121427, GB-A-2153840, GB-A-2153384, U.S. Pat. No. 4,517,306, U.S. Pat. No. 4,539,304, U.S. Pat. No. 4,539,305, U.S. Pat. No. 4,547,472, GB-A-2166972, U.S. Pat. No. 4,579,831, U.S. Pat. No. 4,608,356 and EP-A-201856.
The product recovered from the usual methods used to prepare zeolite L is a fine sized crystalline material. Several of the uses as catalysts or molecular sieves require a product in a size range substantially larger than the size of the product recovered from the preparation processes of the prior art. To meet this demand, various binders are used in forming steps to prepare aggregates containing zeolite L as the principal ingredient such as pellets, extrudates, or tablets. These aggregates lose some of their activity per unit weight since the binder has a different and low activity and acts-as a diluent of the molecular sieve activity and the conventionally-bound aggregates frequently do not have sufficient crushing strength, particularly when they contain the cylindrical zeolite L crystallites as described in EP-A-96479. In addition particles made using alumina as binder are susceptible to blocking of the zeolite pores, as a result of alumina migration. It is therefore highly desirable to develop a method of preparing binderless aggregates having a particle size suitable for catalyst or sieve systems and possessing good attrition resistance.
The prior art has developed processes of producing binderless sieve aggregates from silica and alumina starting materials such as silica-alumina catalysts and clay. Unfortunately the products produced by these processes, especially where clay is used as a starting material, generally have very poor attrition resistance and thus rapidly break during use into unsuitable powders which must be replaced.
U.S. Pat. No. 3,650,687 describes processes for the preparation of binderless zeolite particles including zeolite L, in which an alumina silicate clay is slurried with an alkali silicate, spray dried to form particles of the desired finished size and then treated with an alkali and aged to convert the particles to zeolite. In an alternative, a hydrated clay is slurried and spray dried to form particles., then calcined and reacted with the other components necessary to form a zeolite. Thus zeolite is only formed after the final particles have been formed. Predictable formation of zeolite having optimum catalytic properties may be difficult under such circumstances.
Also spray drying can be used only to give small particles, typically of 100 to 400 microns, which are only suitable for fluidised beds whereas reactors usually need particle sizes of at least 0.8 mm, preferably at least 1.5 mm and typically 3 mm.
GB-A-1316311 describes binderless zeolite particles which may be of zeolite L, and which are formed by pelleting, crush and repelleting repeatedly to give products of the desired strength. This is a time-consuming procedure which is costly and can damage the zeolite crystals.
GB-A-2109359 describes the preparation of zeolite 3A and 4A binderless particles in various processes in which kaolin clay and sodium hydroxide (in some cases with zeolite) are formed into beads and then reacted with further sodium hydroxide to form zeolite 4A (sodium form) which is exchanged to form zeolite 3A (potassium form). It is clearly stated that direct formation of a potassium zeolite is not possible in this process.
GB-A-2160517 describes the formation of so called preformed zeolite particles, which may be zeolite L particles prepared from a starting material, which may be a synthetic zeolite but must have a silica/alumina ratio lower than the product. The starting material is reacted with a silica material and an alkali to form the product. To form zeolite L either zeolite 3A, kaolin or a silica-alumina starting material is used. The products are necessarily more silica rich than the starting zeolite. This process has practical handling problems in treating particles with a silica containing material.
The prior art systems are either unsuitable for providing binderless particles of zeolite L having high catalytic or absorbent activity, or result in particles of inadequate strength to be suitable for handling in practical applications or are impractical for large scale operation.
EP-A-284206 (counterpart to U.S. Pat. No. 5,486,348) describes the preparation of binderless zeolite L particles involving the reaction of an alkaline solution comprising a source of alumina with particles formed from silica and 0 to 95 wt % of preformed zeolite L crystallites to convert the silica binder to zeolite L and obtaining the desired particles by crystallisation of the reaction mixture.
The present invention concerns a method of preparing binderless zeolite L particles with further enhanced catalytic performance and/or capacity over those prepared by the process of EP-A-284206 and with very high mechanical strength, for example against attrition. The invention also provides certain novel binderless zeolite L particles.